There are many practical situations where it is advantageous to learn certain physical characteristics of a material medium without damaging the medium itself. For example, a civil engineer that is building a bridge might want to test the concrete blocks to be used in the bridge to make sure that they meet specifications before installing them, or a farmer might want to learn the moisture content of soil before deciding how much water needs to be used for irrigation. A well established technique for noninvasive material testing is known as impedance spectroscopy, also referred to as dielectric spectroscopy.
FIG. 1 depicts, in schematic form, a simplified implementation of impedance spectroscopy in the prior art. The material to be tested is placed between the two parallel plates of an electrical capacitor, depicted as capacitor plate 120-1, and 120-2 in the figure. This arrangement makes it possible to subject the material to an electric field by applying a voltage to the two plates.
The response of a material to an applied electric field is, in general, a consequence of the material's physical make-up. Such response is usually characterized as a parameter known as the “electrical permittivity” (hereinafter just “permittivity”). For example, the permittivity of soil varies substantially as a function of the amount of moisture in the soil. Similarly, the permittivity of concrete and other construction materials reflects both the composition of the material as well as its condition. It is well known that concrete might exhibit degradation due to environmental factors, and a measurement of the permittivity of concrete can provide information about the concrete's age and integrity.
In FIG. 1, the material to be tested is subjected to a time-varying electric field by connecting the capacitor plates to an “alternating-current” (AC) voltage source, depicted as AC voltage source 130. Note that identification of a voltage source as an “alternating-current” source is, strictly speaking, a misnomer because a voltage source generates a specific voltage, not a current; however, the “AC” abbreviation is commonly used in the art to refer to any source of electrical voltage or current whose output varies as a sinusoidal function of time. An AC source is characterized by an amplitude, which can be expressed in Volts for a voltage source, and a frequency, which can be expressed in Hz.
When AC voltage source 130 applies a specific AC voltage to the capacitor plates, an AC current flows through the circuit, depicted by an arrow as AC current 140 in the figure. The current can be characterized by an amplitude, which can be expressed in Amperes, and a phase, which can be expressed in radians or in angular degrees. The combination of amplitude and phase can be represented as a single quantity by a complex number. It is well known in the art how to use complex numbers for characterizing the behavior of AC circuits and devices.
The complex value of AC current 140 is strongly dependent on the permittivity of material 110. Therefore, measuring the complex value of AC current 140 provides information about the permittivity of the material and, thereby, the condition of the material itself.
Because the voltage applied by AC voltage source 130 and the AC current 140 are both conveniently represented by complex numbers, the permittivity is also conveniently represented by a complex number. In particular, the imaginary part of the permittivity reflects the fact that some of the energy generated by AC voltage source 130 is dissipated inside material 110, and the value of the imaginary part reflects the extent of the dissipation (also commonly referred to as “loss”). Conversely, the real part of the permittivity reflects the fact that some of the energy generated by AC voltage source 130 is stored, without loss, inside material 110. Such stored energy is released by the material at a time different from the time when it was absorbed. The released energy can flow back to AC voltage source 130; or it can be dissipated inside the material, thereby reducing the energy that AC voltage source must deliver.
A feature of an AC voltage source is, of course, that the instantaneous voltage oscillates sinusoidally. As the instantanteous voltage generated by AC voltage source 130 oscillates, the flow of energy out of (or into) AC voltage source 130 is different at different points in the oscillation cycle, and the details depend on the relative strengths of the two physical phenomena, storage and dissipation, characterized by the real and imaginary parts of the permittivity. The full complex value of the AC current 140 reflects these details and provides the necessary information for calculating both the real part and the imaginary part of the permittivity.
Inside a material, energy storage and dissipation are generally mediated by different underlying physical phenomena. Therefore, to achieve a complete characterization of the dielectric properties of a material, it is important to measure the full complex permittivity by independently measuring both the real part and the imaginary part.
Impedance spectroscopy, as depicted in FIG. 1, is advantageous because it yields an estimate of both real and imaginary parts of the complex permittivity. However, the requirement that the material be placed between the plates of a capacitor is a significant obstacle to noninvasive testing of materials in the field. For example, the civil engineer that wants to periodically test the conditions of the concrete in a bridge, after it's built, cannot easily collect samples of the concrete for testing in a lab equipped with an impedance spectrometer without damaging the bridge. Similarly, a farmer that wants to know the moisture contents of soil at a certain depth below the surface, would very much like to be able to do so without having to dig a hole to collect a soil sample. A noninvasive way of remotely testing a material in the field would be very advantageous.
FIG. 2 depicts a system known in the prior art for performing noninvasive material testing. The system is described by H. E. Nilsson, U.S. Published Patent Application 2010/0090802 A1 (hereinafter “Nilsson”). The system takes advantage of so-called Radio-Frequency IDentification (RFID) technology which provides simple, small, low-cost RFID tags that can be queried by an RFID reader via a radio signal.
In the depiction of FIG. 2, an RFID tag 220 is embedded inside the material 210 to be tested. The RFID tag is equipped with a radio antenna 230 for receiving and transmitting Radio-Frequency (RF) signals. An RFID reader 240 located outside of the material to be tested transmits RF signal 255 via its radio antenna 250. The RF signal 255 penetrates the material 210 and reaches antenna 230. The RFID tag responds to the RF signal by transmitting a second RF signal 265, which is received by the RFID reader.
Many techniques are known in the art for RFID tag 220 to generate the response RF signal 265; however, the technique known as backscatter modulation is often preferred because it leads to a simple and inexpensive design for the RFID tag. With backscatter modulation, RF signal 255 is simply reflected by antenna 230, to generate RF signal 265. Such reflection can be accomplished by connecting antenna 230 to an electronic component that reflects back the signal received by the antenna. Electronic circuitry inside the RFID tag can control how the reflecting component reflects the signal, such that the reflectivity of the component can be modulated in accordance with information that the RFID tag wants to convey to the RFID reader.
Through backscatter modulation, the RFID tag accomplishes an important goal: the reflected RF signal 265 is modulated with a unique pattern that uniquely identifies the reflected RF signal 265 as originating from RFID tag 220. This unique pattern enables the RFID reader to extract the reflected RF signal 265 from the clutter of other reflected signals that might be reflected by other objects in the vicinity, including, possibly, other RFID tags.
The ability to extract RF signal 265 from unwanted clutter, allows RFID reader 240 to obtain a good estimate of the signal strength of RF signal 265. Such signal strength is strongly affected by the presence of material 210, and by the material's response to RF signals. In particular, the permittivity of material 210 has a strong influence on ease of propagation of RF signals through the material, such that the signal strength of RF signal 265, as received by RFID reader 240 carries information about the permittivity of material 210.
Unfortunately, as mentioned in previous paragraphs, the permittivity of a material is, in general, a complex number characterized by a real part and an imaginary part. The single measurement of signal strength performed by RFID reader 240 in FIG. 2, provides valuable information about the permittivity of material 210, but it is fundamentally impossible to derive the values of two independent unknown quantities from a single measurement. Therefore, it is impossible, with the system of FIG. 2, to independently estimate the values of both the real part and the imaginary part of the permittivity from just the one measurement of signal strength of RF signal 265. As such, the system of FIG. 2 cannot provide a full characterization of the permittivity of material 210. Clearly, there is a need for a noninvasive way of estimating both real and imaginary parts of the permittivity of a material in the field.